Residual oils, including reduced crude oils, atmospheric tower bottoms, topped crudes, vacuum resids and heavy oils, are considered difficult to catalytically crack to form high yields of gasoline plus lower and higher boiling hydrocarbon fractions because of the deposition of large amounts of coke deposited on the catalyst during the cracking. Furthermore, metal contaminants in the heavy oil fractions of crude oil are deposited on and/or in the pores of the catalyst, thereby further poisoning and inactivating the catalyst so employed. At one time, resid oils were regarded as distress stocks by the petroleum industry.
Largely within the last ten years, methods have been devised for the simultaneous conversion of both the high and low boiling components contained in residual oils with high selectivity to gasoline and lighter components and with low coke production. These conversion processes have been made possible largely because of the use of two-stage catalyst regeneration processes. In the first stage of such regeneration processes, catalyst particles, which have deposited on them hydrocarbonaceous materials such as coke, are regenerated under conditions of oxygen concentration and temperature selected to particularly burn hydrogen associated with hydrocarbonaceous material. These conditions result in a residual level carbon left on the catalyst and production of a CO-rich flue gas. This relatively mild first stage regeneration serves to limit local catalyst hot spots in the presence of steam formed during the hydrogen combustion so that formed steam will not substantially reduce the catalyst activity. A partially regenerated catalyst is recovered from the first regenerator substantially free of hydrogen. The hydrogen-free catalyst comprising residual carbon is passed to a second stage, higher temperature regenerator where the remaining carbon is substantially completely burned to CO.sub.2 at an elevated temperature within the range 1400.degree. F. up to 1800.degree. F.
The second stage high temperature regenerator is designed to minimize catalyst inventory and catalyst residence time at the high temperature while promoting a carbon burning rate to achieve a carbon on recycled catalyst less than 0.5, preferably less than 0.1 and most preferably less than 0.05 weight percent. This second stage regeneration is conducted in the presence of sufficient oxygen to substantially burn residual carbon deposits, CO and produce CO.sub.2 -rich flue gas.
The regenerated catalyst is withdrawn from the second stage and charged to the riser reactor at a desired elevated temperature and in an amount sufficient to vaporize the hydrocarbon feed. The catalyst particles are at a temperature typically above 1400.degree. F. and at least equal to the pseudo-critical temperature of the hydrocarbon feed comprising the residual oils. The catalyst particles are at a temperature such that, at the selected catalyst feed rate and hydrocarbon feed rate, the vaporizable components of the hydrocarbon feed are substantially completely vaporized in the riser reactor whereby thermal and catalytic cracking of the feed is accomplished. Resid cracking processes employing this process are described in U.S. Pat. Nos. 4,331,533 and 4,332,674.
The above-described processes, then, made possible catalytic cracking of resid oils by solving what had been thought to be two temperature limitations on the process, namely (1) metallurgical limits of the regeneration equipment and (2) thermal stability of the catalyst. Those processes were able to extend the temperature of regeneration up to at least 1800.degree. F. without unduly impairing catalyst activity, and could be performed in equipment or apparatus capable of withstanding the severe temperature operations contemplated by those processes. Removing such temperature restrictions on the regeneration step made feasible the high temperature conversion processes which were required to convert residual oils.
These processes are not without some temperature restraints, however. For instance, the cracking catalysts typically used lose their catalytic properties when subjected to sufficiently high temperature for extended periods. As the coke-formation potential of the feed increases, e.g. because of increased Ramsbottom carbon content, the amount of coke deposited on each particle of catalyst, all other factors being held constant, will increase, leading to greater coke burning in the regeneration stage and thus hotter catalyst entering the riser. It has been found, for example, that as the Ramsbottom carbon content of the feed approaches around 4 to 5, the regenerated catalyst is reaching temperatures which begin to produce problems for the conversion process.
One solution to this problem might be a direct dissipation of heat from the cracking/regeneration cycle. This solution is unattractive for two reasons. First, techniques for direct heat dissipation from the cracking/regeneration cycle typically require apparatus which is expensive to produce, install and maintain. Second, direct heat dissipation represents a waste of the heat value of the hydrocarbon feed.
Accordingly, it would be advantageous to provide a hydrocarbon conversion-catalyst regeneration process operable with high carbon content feeds at high feed rates and/or which is easy to retrofit on existing equipment and/or which increases the percentage of resid in the acceptable feed.
It is an object of this invention to provide a process for conversion of hydrocarbons and regeneration of cracking catalyst having means for reducing the temperature of the regenerated catalyst.
It is another object of this invention to provide a process for conversion of hydrocarbons and regeneration of cracking catalysts which is usable with hydrocarbon feeds having a high carbon content.
It is still a further object of this invention to provide a cracking catalyst regeneration process including cooling means which are easily installed and maintained in existing facilities.